I killed a player, had my primary gun in hand and double clicked his AK74 to put it on my back for me to take away. The gun moved from his inventory to mine and went on my back. However when I hid the players body, the gun on my back disappeared. When I went to put my primary gun on my back later it fell on the floor. The AK74 for was no longer there for me to access but I could not put any other primary gun on my back due the slot being taken by the "invisible gun."